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The Evolution of Low Mass Stars to Helium Ignition
Overview
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Look at how structure of a star changes
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How do its abundances change?
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We will look at particular phases in the
evolution:
Main sequence
 The giant branch
 Helium ignition
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The Hertzsprung-Russell Diagram
The Main Sequence
1Mʘ, Z=0.02
Nuclear burning
The Main Sequence - Convection
5Mʘ, Z=0.02
Nuclear burning - convection
Shell burning
Shell burning II
Shell burning III
The Giant Branch
First Dredge-up
First Dredge-up II
First Dredge-up III
First Dredge-up IV
First Dredge-up V
The Luminosity Bump
Problems!
Lithium
Carbon
12C/13C
Nitrogen
Oxygen
Sodium
Gratton et al.
(2000)
-1
0
1
2
Log L/Lsol
3
Extra Mixing!
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Material that has undergone nuclear burning
reaches the surface

This is not convection!!!
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Some additional mixing process is at work
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We need to circulate material between the
burning shell and the base of the convective
envelope...
Helium ignition
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Eventually the star becomes hot enough to
ignite He
In stars of >2 solar masses, this happens nondegenerately
For the low-mass stars, the ignition occurs
under degenerate conditions...
...we get a Helium Flash
Degenerate Cores

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As core mass grows, so does the density
Core becomes electron degenerate – e's can't
be forced into the same states
Equation of state P = P()
There is no longer any temperature
dependence
No thermostatic control...
Helium Flash

At 108 K, He begins to burn
3  12C + 

Burning releases energy

The temperature gets hotter

The burning gets faster

More energy is released....
Off centre ignition
Huge Energy Release!
But none of it reaches the surface :-(
On to core He-burning